Optical Studies on Functionalized Graphene Systems
Ajayan, Pulickel M.
Doctor of Philosophy
Graphene, the ‘wonder material’, has received a lot of attention for its excellent electronic properties. However, the lack of a band gap severely limits its use, especially in optoelectronic applications. Therefore, opening a band gap in Graphene and controllably modifying its band structure has long been the holy grail in the physics of Graphene. Of these methods, chemical functionalization offers the most degrees of freedom in controllably modifying the band structure of Graphene. Graphene Oxide (GO), the most widely studied chemical derivative of Graphene exhibits a host of optical phenomena such as broadband tunable fluorescence, multiphoton-induced absorption and emission etc. and presents an excellent platform for studying the effects of chemical functionalization on the optical properties of Graphene. In the present work, we first deal with the issue of origin of fluorescence in GO. It is argued that the broadband emission arises due to localized states created on the Graphene surface due to presence of functional groups, and not due to quantum confinement. Next, we attempt to find which of the many functional groups in GO contribute the most to the emission intensity. We find that the carbonyl and epoxide functional groups contribute the most to fluorescence. Further, we find that irradiation with a laser causes an enhancement in the PL of multilayered GO sheets by increasing the density of carbonyl functional groups on the basal plane. This interesting phenomenon is proposed to occur due to a reaction between the oxygen-functionalized basal plane and water molecules trapped between the GO multilayers. We have also developed a method for synthesizing large-area Graphene by chemical vapor deposition (CVD) using liquid precursors. This opens up a host of new possibilities for substitutional doping of Graphene by using liquid precursors containing the dopant atoms.
Graphene; Graphene oxide; Optoelectronics; Graphene Band Gap; Graphene Oxide Fluorescence